Method of producing pulp using single-stage cooking with formic acid and washing with performic acid

ABSTRACT

The invention relates to a method of producing pulp with a high brightness using single-stage cooking with formic acid, washing with performic acid and bleaching with oxidizing bleaching chemicals. In performic acid washing, the rapid formation and rapid reactions of performic acid (HCOOOH) are utilitized for modification of residual lignin. The method of the invention provides an ISO brightness exceeding 90% without a chlorine chemicals. The method can be applied to both herbaceous plants and wood.

The invention relates to a method of producing pulp with a highbrightness using single-stage cooking with formic acid, washing withperformic acid and bleaching with oxidizing bleaching chemicals. Inperformic acid washing, the rapid formation and rapid reactions ofperformic acid (HCOOOH) are utilized for modification of residuallignin. The method of the invention provides an ISO brightness exceeding90% without chlorine chemicals. The method can be applied to bothherbaceous plants and wood.

The production of pulp from herbaceous plants (common reed, goat's rue)is described in the publication Laamanen J. & Sundqvist, J., Agrokuiduntuotanto ja kaytto Suomessa [Production and use of agrofibre inFinland]. Final report on the study, Part III, Vaihtoehtoisetkuidutusmenetelmat [Alternative defibration methods], Jokioinen 1996,Agricultural Research Centre, Publication 5 series A, p. 69 to 88.Common reed is cooked using single-stage, two-stage and three-stageMilox methods. In single-stage Milox cooking, a mixture of formic acidand hydrogen peroxide at a maximum temperature of 80° C. is used. In thetwo-stage method, the grass is first cooked in formic acid alone andthen in a mixture of formic acid and hydrogen peroxide. The three-stagemethod first comprises a peroxy formic acid cooking stage, followed bytwo-stage Milox cooking.

After Milox cooking the pulps are bleached with alkaline peroxide. Thepublication states that the common reed pulp reaches its targetbrightness 80 to 85% with alkaline peroxide alone, but, in spite of itslow kappa number, uses more peroxide than Milox pulp produced from e.g.birch chips. The yield of the bleaching is stated to be quite low too,about 80%. The best brightness 83.4% was reached using five peroxidestages and a peroxide consumption of 3.3% calculated on pulp.

As to cooking goat's rue, it is stated that goat's rue Milox pulp can bebleached to a brightness exceeding 85% with alkaline peroxide alone andthat peroxide consumption is in the same range as with common reed Miloxpulp (4%). Four peroxide stages have provided an 84.5% brightness withan about 3% peroxide consumption calculated on pulp.

The summary of the publication states that the best results, both whenusing common reed and when using goat's rue, have been achieved by atwo-stage Milox method first involving cooking in formic acid alone andthen in a mixture of formic acid and hydrogen peroxide at 80° C. for 3hours.

Brightness values exceeding 90% have not been reached with the abovemethods.

Finnish Patent 74750 discloses a method of producing bleached pulp fromlignin-containing cellulose raw material, such as softwood or hardwoodchips. Organic peroxo acids, such as peroxo acids derived from formicacid, are used as defibration reagents. The defibration reagent can beproduced by e.g. adding hydrogen peroxide to formic acid. An alkalinesolution containing hydrogen peroxide is used for bleaching. In theexample of the patent, the highest achieved brightness was 90.3%. Thehydrogen peroxide amounts used were, however, relatively high,preferably 5 to 20% calculated on raw material, and the cooking timeswere long.

It is known from the publication Sundqvist J., Chemical Pulping Based onFormic Acid--Summary of Milox Research, Paper and Timber 78 (1996)3, p.92 to 95, to feed hydrogen peroxide into the first and third cookingstages of multi-stage formic acid cooking. In the method, the chips aretreated with formic acid and a small amount of hydrogen peroxide at 60to 80° C. in the first stage. The main delignification is carried out inthe second stage at a temperature of 90 to 100° C. In the third stage,the pulp is cooked in a solution containing formic acid and hydrogenperoxide. In all three stages, the formic acid content in the cookingsolution exceeds 80% and the cooking times in each stage vary between 1and 3 hours. Liquid circulation is planned such that after the cookedpulp has been washed with acid, the acid enters the third stage, fromwhere it enters the first cooking stage, and from there the secondcooking stage, and then goes further to regeneration. This way hydrogenperoxide and performic acid are used up by the oxidation of dissolvedlignin, and peracid is subjected to impurities decomposing hydrogenperoxide and peracid and causing extra peroxide consumption. Thereturned acids cause the kappa number of the pulp to remain higher thanthat of pulp cooked using pure acids, resulting in increased hydrogenperoxide consumption.

In the method of the invention, the drawbacks of said known methods,i.e. primarily insufficient brightness of non-wood materials, highhydrogen peroxide consumption, and high investment costs, have beeneliminated by utilizing the good delignification characteristics offormic acid at temperatures exceeding 85° C., and preferably at atemperature higher than the normal boiling point of formic acid, and byutilizing the rapid formation and rapid reactions of performic acid withresidual lignin alone. In addition, the number of formic acid ccokingstages can be reduced from two or three-stage to single-stage. In themethod of the invention, the kappa number (20 to 30) of the pulp afterdefibration, washing with performic acid and washing with water issufficient to achieve the final target brightness (more than 90%). Thepulp can be bleached in fewer stages than before and to a higherbrightness than before without chlorine compounds. High brightness isachieved by simultaneously maintaining good viscosity.

The invention relates to a method based on single-stage formic acidcooking for producing pulp with a high brightness, the method comprisingthe stages of:

(a) defibrating cellulose raw material in single-stage cooking withformic acid, possibly together with other organic acids,

(b) removing cooking liquor from the defibrated pulp and washing thepulp with formic acid,

(c) removing acid/acids from the pulp, and

(d) bleaching the pulp by using oxidizing bleaching chemicals.

The method is characterized in that the formic acid washing in stage (b)comprises as the last acid washing stage a stage (b1) in which the pulp,free from cooking liquor and in a medium or high consistency state, iswashed with formic acid containing performic acid produced in situ andpossibly other peracids.

In step (a) the cellulose raw material is defibrated with single-stageformic acid cooking. Formic acid cooking is carried out at a temperatureexceeding 85° C., preferably at a temperature between 105 and 135° C.,most preferably at a temperature between 110 and 125° C. The cookingtime of herbaceous plants can vary between 15 and 80 minutes, preferablybetween 20 and 40 min. A cooking time typical of herbaceous plants ise.g. 20 min. A cooking time typical of hardwood material is 25 to 90min. The method is also suitable for straw and other fibre-basedagricultural waste. The time during which the temperature is raised toreaction temperature, including possible absorption time of cookingchemicals, is 30 to 70 min.

A short single-stage cooking time with formic acid alone allows the useof the most preferable continuous screw reactor technique for grassesand the use of the continuous Kamyr digester and the Super batchdigester for wood chips.

In addition to formic acid, other organic acids can also be used, suchas acetic acid, a small amount of which is inherently formed in theprocess. As a cooking reagent, acetic acid is not, however, as efficientas formic acid.

Cooking liquor is removed from the pulp defibrated in stage (b) byleading it to a screw or pressure press, a vacuum filter, or the like.The removed cooking liquor contains the bulk of the dissolved lignin.Pulp consistency in said pressing or filtering stage can be 20 to 55%,preferably 30 to 50%.

Typically the pulp is then led to an acid washing stage in which morecooking liquor is removed. The pulp is typically acid washed with formicacid, preferably as a counter-current wash in such a way that the acidused for the wash is achieved as return acid from the performic acidwashing stage (b1). The formic acid used in the performic acid washingstage, in turn, is preferably achieved from the regeneration of thecooking liquor. The acid removed from the washing stage is preferablyrecycled to the formic acid cooking in stage (a).

The concentration of the formic acid used in the washing stages istypically 65 to 99%.

The pulp is washed and pressed in several stages, typically 2 to 6stages, preferably in pressure washers. Before treatment with performicacid, i.e. the last acid washing stage, pulp consistency is high,typically 10 to 50%, preferably 20 to 35% dry matter.

When the pulp has been processed as described above and is in a mediumor high consistency state, lignin, hemicellulose, fats and metals havebeen removed from it. The removal of these intensifies the reaction ofhydrogen peroxide with formic acid to be performed in the next stage,i.e. the formation of peracids, thus enhancing the reactions of residuallignin. Only residual lignin, and a very small content, is present ofthe lignin in the pulp processed in this way. Otherwise the performicacid of stage (b1) would be taken up by unnecessary reactions and thehydrogen peroxide would decompose or would, together with metals, causeformation of radicals and a decrease in viscosity.

It should also be noted that fats and fatty acids use performic acid toform epoxides. The formation of these can be reduced as formic acid assuch is able to hydrolyze fats and fatty acids which, in accordance withthe present invention, are separated with the cooking liquor, andperacid/hydrogen peroxide is only used in pulp free from fatty acids. Ifthe performic acid performs reactions unnecessary to the process,hydrogen peroxide consumption rises remarkably high, as in previousmethods. This drawback has been eliminated in the method of theinvention by treating the pulp in the above stages (a) and (b) in such away that only a small amount of residual lignin remains for theperformic acid reaction.

The significance of acid washings to vegetable nutrients is importantsince all nutrients arriving at the mill together with the plants can betransferred to the cooking liquor during acid cooking. The lignin,concentrated in connection with the regeneration of the cooking liquorduring evaporation, and the hemicellulose phase comprise the nutrients,and the acid is evaporated to be recycled. The nutrients can be utilizedeither as fuel ash fertilizers for non-wood fields or as nutrient saltsin hemicellulose fermentation.

In stage (b1), the medium or high consistency pulp, free from cookingliquor, is treated with formic acid containing performic acid preparedin situ. The performic acid is prepared immediately before use in situe.g. by adding hydrogen peroxide to formic acid. In practicalimplementations of the method of the invention, the performic acidtreatment is preferably performed in connection with acid washing as thelast acid washing stage in a peracid washer.

The formic acid needed in performic acid washing is preferably obtainedfrom the regeneration of the cooking acid as a pure regenerated acid.Residual peracid is preferably led in counter current to the previouswashing stage. The pulp exiting the last acid washing, i.e. thetreatment with performic acid, is pressed to as high a dry mattercontent as possible, whereby the peracid remaining in the pulp finishesits reaction and the pressed cooking liquor is used in previous washingstages and from there in counter current as cooking liquor.

The amount of hydrogen peroxide used in the preparation of performicacid is 0.01 to 1.5%, preferably 0.2 to 1.0%, calculated on the amountof cellulose raw material. The formation and reaction time of peracid is1 to 20 min, preferably 5 to 15 min. The reaction time of peracid issuitable for the treatment times of typical washers and compressors.Pulp consistency is 10 to 50%, preferably 20 to 35%, and treatmenttemperature is 50 to 90° C., preferably 60 to 80° C. The hydrogenperoxide is added to concentrated formic acid or to the pulp containingformic acid. The typical concentration of formic acid is 65 to 99%,preferably 70 to 90%.

The formation of performic acid in concentrated formic acid is a rapidreaction. As early as 2 to 5 minutes from the feed of hydrogen peroxideto e.g. 70° C. formic acid, the performic acid concentration has reachedits peak. When hydrogen peroxide and the formed performic acid are fedin accordance with the present invention to medium or high consistencypulp in the washing and pressing stage, the amount of performic acidrelative to the amount of residual lignin is at its highest. Theperformic acid can then optimally react with the residual lignin andhydrogen peroxide consumption is minimized. The residual lignin can beoptimally modified for bleaching.

The performic acid treatment time is usually the treatment time of atypical washing stage.

In stage (c), acid is removed from the pulp in the simplest way bywashing with water at an elevated temperature of 80 to 120° C. and/orusing a vacuum. Acid is usually removed in several stages. Aconventional equipment is used for the washing. In washing the pulp withwater, the amount of water used relative to the amount of pulp is sosmall that the acid obtained from the water wash is an acid concentratedin several counter current stages and having a concentration of 30 to50%.

Washing the pulp with water to recover acids in stage (c) is easy, sincelignin, hemicellulose and fatty acids have been well washed from thepulp in the acid washing stage (b). Filtration and water removal istherefore rapid. In contrast, when cooking herbaceous plants withalkaline cooking chemicals, foaming and fine silicate crystals disturbthe filtration of water washings. Acid cooking chemicals do not showsimilar disadvantages. In stage (d), the pulp is bleached by usingoxidizing bleaching chemicals. Bleaching is preferably performed byusing alkaline hydrogen peroxide bleaching. Acid-hydrogen peroxidebleaching or other oxidizing bleaching sequences can also be used.Bleaching is preferably performed as pressure bleaching. A typicalbleaching temperature is 80 to 125° C., preferably 100 to 115° C., andbleaching pressure 0.5 to 10 bar, preferably 2 to 5 bar. Bleaching timemay vary between 25 and 120 min, preferably from 30 to 60 min. When onlyperoxide is used as bleaching chemical, the typical amount of hydrogenperoxide is 3 to 6.5%.

One pressure bleaching typically provides a high brightness, 81 to 86%,and two stages full brightness, 87.5 to 90%, three stages an ISObrightness exceeding 90%, and four stages an ISO brightness of 92%. Onestage typically takes up 60 minutes of bleaching time at the temperaturerange of 100 to 110° C.

In the method of the invention it is essential that the viscosity of thepulp remain high. This can be affected by efficient mixing conditionsfor acid chemicals and by using small amounts of acid chemicals at atime. The viscosity of herbaceous plants usually remains naturally highowing to the silicates; contained by the plants, the silicates acting asnatural protective agents for viscosity. Magnesium compounds and/ordiethylene triamine penta acetic acid (DTPA) or ethylene diamine tetraacetic acid (EDTA) can be used as additional protective agents ifneeded, and these usually have to be added when using wood as rawmaterial.

Maintaining viscosity in bleaching is highly dependent on pH. Bleachingis successful when pH is over 10.5, but when Mg compounds are used asstabilizers and pH is higher than 11, these compounds becomeprecipitated with silicate and do no longer act efficiently asprotective agents for viscosity.

Non-wood material contains large amounts of hemicellulose. In the methodof the invention, the hemicellulose content is easily adjustable suchthat its too high concentrations do not disturb the production ofcellulose or paper and do not impair the quality of paper causing weakerlight-scattering properties, yellowing and brittleness, i.e. drawbacksthat should be avoided particularly in high-grade paper.

Owing to its high brightness, the pulp produced by the method of theinvention can be used for the production of high-grade paper.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates the method of the invention.

The process diagram shown in the attached FIGURE illustrates theoperation of the method of the invention.

The process diagram shown in the FIGURE illustrates how raw material(10) is fed into a cooking reactor (1) in which the delignificationchiefly takes place. As cooking chemical in said reaction, formic acidoriginating from counter-current acid washings (3) and further fromperacid washing (4) is used. The cooking liquor (11) used is removedfrom pulp (2) by e.g. pressing. Peracid washing (4) is performed usingacid from regeneration (12) and hydrogen peroxide (15) mixed either tosaid acid or directly to the pulp. The acid is removed from the pulp(5-6) e.g. by washing with water. Clean water for the washings (5-6) isobtained from regeneration (14) and the washing is performed in countercurrent. Bleaching chemicals (19) are fed into bleaching (7) and thebleached pulp (18) is washed (8) with water (17).

The following examples illustrate the method of the invention. In theexamples, the ISO brightness was measured using the SCAN-C 11:75standard, the kappa number was measured by the SCAN-C 1:77 standard andthe viscosity in copper ethylene diamine solution by the SCAN-CM 15:88standard.

EXAMPLE 1

(A) Pulp Defibration Stage

Non-fractionated reed canary grass (710 g, humidity 12.5%), cut up in 5cm bits, was added to a pre-heated 10-liter pressure reactor. Formicacid (82%, 80° C.) was aspirated by a vacuum into the reactor so as togain a formic acid (containing the humidity of the raw material) andcommon reed ratio of 4.5:1. The initial pressure in the reactor wasraised by nitrogen to the value 1 bar. The temperature in the reactorwas raised to 117° C. within 50 minutes, during which time the formicacid was absorbed in the raw material to be treated. The reactor waskept at 117° C. for 20 minutes.

Cooking liquor was removed from the pulp by vacuum filtration, and thepulp was washed with 85% formic acid in four stages. The pulp was thentreated with performic acid/formic acid as the last acid washing stage.The performic acid was prepared immediately before use by adding 0.75%hydrogen peroxide, calculated on raw material, to the formic acid. Pulpconsistency in the performic acid washing was 19%. The acid was washedfrom the pulps with warm water in multiple stages, resulting in a kappanumber of 21.8 and a viscosity of 904. The washing and filtering stageslasted for a total of 7 minutes. The formation of peracid to its maximumconcentration at 66° C. took 3 minutes.

(B) Bleaching of Pulp

Pulp (247 g dry matter, consistency 15%), which was prepared in theabove manner and sorted, was fed into a bleaching reactor. Bleachingtemperature was 92 to 102° C. (raised within 20 minutes), bleaching time40 minutes and bleaching pressure 4 bar.

One bleaching stage resulted in an 81% ISO brightness with a viscosityof 774. No additives were used in the bleaching for the protection ofviscosity. A second alkaline peroxide stage (87° C., 2h) resulted in abrightness of 87.6. A third alkaline pressure peroxide bleaching (105°C., 40 min, temperature raised within 20 min) resulted in a 90% ISObrightness and a fourth stage involving alkaline pressure peroxidebleaching (110° C., 30 min, temperature raised within 30 min) resultedin a 92.0% ISO brightness.

Total hydrogen peroxide consumption in the four bleaching stages was6.3% of the amount of pulp with a pH range of 12 to 9.5.

EXAMPLE 2

A cooking reactor was filled with the same raw material in the same wayas in example 1. Cooking temperature was 122° C. and cooking time 19minutes (absorption time and temperature raising time was 40 min). Inpulp peracid treatment, 1% hydrogen peroxide of the amount of pulp wasused and the treatment temperature was 68° C. Pulp consistency inperformic acid washing was 19%. The kappa number of the pulp afterperacid washing and water washings was 25.0 and viscosity 916.

The pulp was bleached at 105° C. with an alkaline pressure peroxide(amount of peroxide 4% of amount of pulp), brightness being 86.6% ISOafter the first stage and 90.0% ISO after the second stage (110° C., 2%hydrogen peroxide). Total hydrogen peroxide consumption in the twobleaching stages was 4.3% of the amount of pulp with a pH range of 10 to10.5. Final viscosity was 792. Magnesium sulphate (0.5%) was used toprotect viscosity.

EXAMPLE 3

Birch chips (dm 85%) were cooked in 82% formic acid at a temperature of120° C. for 26 minutes (absorption time and temperature raising time 90min). Peracid treatment was performed using formic acid to which 0.5%hydrogen peroxide had been added. Pulp consistency in peracid washingwas 20%. Pulp kappa number after peracid washing and water washings was27.5.

The pulp was bleached with alkaline pressure peroxide bleaching at 105°C. for 35 minutes (temperature raising time 30 min) at a pressure of 4.5bar, the final pH being 9.0. Brightness in the first stage was 67.5% ISOand viscosity 1282. The second peroxide pressure bleaching was performedat 110° C. for 60 minutes (temperature raising time 15 min), the finalpH being 10.5. After the second bleaching, brightness was 90.1% ISO andviscosity 1101. The third peroxide bleaching stage was performed at 110to 115° C. for 30 minutes (temperature raising time was 50 min).Brightness was 91.8% and viscosity 1038. In all stages DTPA (0.2%) andMgSO₄ (0.5%) were used.

Total hydrogen peroxide consumption in the three bleaching stages was4.3% of the amount of pulp with a pH range of 9.2 to 10.4.

What is claimed is:
 1. A method based on single-stage formic acidcooking for producing pulp with a high brightness, the method comprisingthe stages of:(a) dignifying cellulose raw material in single-stagecooking with formic acid, possibly together with other organic acids, ata temperature of 105-135° C., (b) removing cooking liquor from thedelignified pulp and washing the pulp with formic acid using amultiple-stage washing sequence, (c) removing the formic acid andpossibly other organic acids from the washed pulp, and (d) bleaching thepulp by using oxidizing bleaching chemicals, wherein the last formicacid washing stage in the multiple-stage washing sequence of stage (b)comprises washing the pulp, free from cooking liquor and in a medium orhigh consistency state, with formic acid containing performic acidproduced in situ and possibly other peracids.
 2. A method as claimed inclaim 1, wherein the performic acid used in the last formic acid washingstage of stage (b) is prepared immediately in situ or immediately beforeuse by adding hydrogen peroxide or ozone to formic acid.
 3. A method asclaimed in claim 2, wherein the amount of hydrogen peroxide used for thepreparation of performic acid in the last formic acid washing stage ofstage (b) is 0.01 to 1.5% of the amount of cellulose raw material.
 4. Amethod as claimed in claim 3, wherein the amount of hydrogen peroxideused for the preparation of performic acid in the last fonnic acidwashing stage of stage (b) is 0.2 to 1% of the amount of cellulose rawmaterial.
 5. A method as claimed in claim 1, wherein the performic acidtreatment in the last formic acid washing stage of stage (b) isperformed at a temperature of 50 to 90° C.
 6. A method as claimed inclaim 5, wherein the pulp is washed in stage (b) in a counter-currentwash using as washing liquid formic acid which is recylced from theperformic acid treatment in the last formic acid washing stage of stage(b) and which contains possibly other organic acids, residual peracidand other peracids.
 7. A method as claimed in claim 5, wherein theperformic acid treatment in the last formic acid washing stage of stage(b) is performed at a temperature of 60 to 80° C.
 8. A method as claimedin claim 1, wherein pulp consistency in the last formic acid washingstage of stage (b) is 10 to 50% dry matter.
 9. A method as claimed inclaim 8, wherein pulp consistency in the last formic acid washing stageof stage (b) is 20 to 35% dry matter.
 10. A method as claimed in claim1, wherein the bleaching in stage (d) comprises alkaline hydrogenperoxide bleaching or combined oxygen and hydrogen peroxide bleaching.11. A method as claimed in claim 10, wherein the bleaching in stage (d)comprises multiple stages to provide a target brightness.
 12. A methodas claimed in claim 11, wherein said bleaching in stage (d) comprisesfrom 1 to 4 stages.
 13. A method as claimed in claim 12, wherein saidbleaching in stage (d) comprises 2 to 4 stages.
 14. A method as claimedin claim 10, wherein said bleaching in stage (d) is pressure bleaching.15. A method as claimed in claim 1, wherein the delignifing stage (a) isperformed at a temperature of 110 to 125° C.